Surface emitting semiconductor laser

ABSTRACT

A method of fabricating a surface emitting semiconductor laser includes a first step of forming, on a substrate, multiple monitor-use semiconductor layers having stripes radiating from a center of the substrate, and a laser portion that includes semiconductor layers and is located on the periphery of the multiple monitor-use semiconductor layers, a second step of monitoring oxidized conditions on the multiple monitor-use semiconductor layers when a selectively oxidized region is formed in the laser portion, and a third step of controlling oxidization of the selectively oxidized region on the basis of the oxidized conditions thus monitored.

This is a Division of application Ser. No. 10/384,607 filed Mar. 11,2003. The entire disclosure of the prior application is herebyincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface emitting semiconductor laserused as a source for optical information processing, opticalcommunications, optical recording and image forming. The presentinvention also relates to a method and apparatus for fabricating such asurface emitting semiconductor laser. More particularly, the presentinvention relates to a technique of accurately defining an aperturesurrounded by a selectively oxidized portion of a current confinementregion.

2. Description of the Related Art

Recently, there has been an increased demand for a surface emittingsemiconductor laser capable of easily realizing an array of sources inthe technical fields of optical communications and opticalinterconnections. Such a laser is also called vertical-cavitysurface-emitting laser diode (VCSEL).

The surface emitting semiconductor laser is categorized into a protoninjection type with a gain waveguide structure, and a selectiveoxidization type with a refractive ratio waveguide structure. Nowadays,the latter is getting the mainstream.

Generally, the selective oxidization type semiconductor laser has alaser portion of a mesa structure (laser-use mesa). A current narrowingor confining region formed by a selectively oxidizing part of an AlAslayer or AlGaAs layer is formed in the vicinity of the active region ofthe mesa. The current confinement layer has an increased resistivity anda reduced refractive index. This results in an optical waveguide path.

The degree of dimensional accuracy of the non-oxidized region surroundedby an aperture formed in the current confinement layer and defined bythe selectively oxidized region is a very important factor thatdetermines the device performance.

The threshold current of laser and the transverse oscillation modegreatly depend on the diameter of the aperture.

A proposal to solve the above problems is described in JapaneseUnexamined Patent Publication No. 2001-93897. The proposal forms alinear stripe pattern that is formed on a substrate and has the samecomposition as that of a pattern of the laser portion having a mesashape formed on the same substrate. The linear stripe pattern is used asa sample for monitoring. In an oxidization step, the degree of advanceof the oxidization reaction on the monitor sample is monitored, andoxidization of the laser portion is controlled based on the monitoreddegree of advance. The proposal utilizes a phenomenon such that thereflectance of an AlAs layer in the monitor sample becomes higher asoxidization thereof advances. Light is projected onto the stripe-likemonitor sample, and the reflectance thereof is monitored.

FIG. 13 is a graph of a relation between light projected onto thestripe-like monitor sample and its reflectance, and FIG. 14 is a graphof a method for controlling oxidization using the conventional monitorsample. Light of a selected wavelength is extracted from the lightprojected onto the monitor sample, and reflected light is monitored. Asshown in FIG. 14, an AlAs layer of the monitor sample has reflectancevalues r1 and r2 when oxidization thereof starts (time t1) and ends(time t2), respectively. Oxidization of the current confinement layer iscontrolled by detecting the reflectance obtained at times t1 and t2 orits variation.

However, the proposal described in the above-mentioned publication hasthe following problems to be solved. The monitor-use sample has a widthnarrower than the diameter of the mesa of the laser portion (the outsidediameter for a cylindrically-shaped mesa) and a stripe-like patternformed on the substrate on which stripe lines are arranged with aconstant period. The monitor sample is defined by anisotropic etching sothat it can be simultaneously formed with the mesa of the laser portion.However, the monitor samples of laser devices have considerabledispersion of the line width due to the actual etching conditions.Sometimes, the sidewall (stripe edge) of the monitor sample is notvertical but inclined. When these faulty monitoring samples are subjectto oxidization, optical diffraction may take place and prevent accuratemeasurement of variation from the reflectance r1.

FIG. 15 shows a graph of a relation between reflectance vs. oxidizationtime for the monitor sample. It is difficult to accurately identify theoxidization initiating time (circle of broken line in FIG. 15) fromvariation in the reflectance. Further, there is a dispersion of thestripe line width. Thus, the average reflectance r1 obtained from thestripe pattern fluctuates. This makes it difficult to accurately measurethe reflectance r1 and detect the oxidization initiating time t1.

If tracking of the oxidized condition on the monitor sample includeserror, it will be difficult to reproduce the aperture in the currentconfinement region as designed at the time of stopping the oxidizationreaction. This degrades the production yield and prevents costreduction. There is another disadvantage. After the step of oxidizingthe current oxidization layer, an insulation film and a metal electrodeare photolithographically formed. In these processes, the resist filmsmay not be deposited on the substrate evenly. Resist is dropped on thesubstrate that is being rotated, the stripe pattern restricts movementof the resist and prevents the resist from smoothly moving towards theends from the center of the substrate. This may prevent evenness in thefilm thickness of resist. The unevenness degrades the accuracy of anoutgoing aperture formed in the metal electrode and affects alignment ofthe outgoing window with the aperture in the current confinement layerand the optical axis of the laser portion.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand provides a surface emitting semiconductor laser and a method offabricating the same.

According to an aspect of the present invention, a method of fabricatinga surface emitting semiconductor laser has the steps of: forming a laserportion of semiconductor layers and at least two kinds of monitor-usesemiconductor layers on a substrate, the laser portion including a firstreflection mirror layer of a first conduction type, an active region, aIII-V semiconductor layer containing Al and a second reflection mirrorof a second conduction type, the at least two kinds of monitor-usesemiconductor layers having oxidizable regions having different shapes;etching the laser portion so as to form a mesa on the substrate in whicha side surface of the III-V semiconductor layer contained in the laserportion is exposed; starting oxidization of the III-V semiconductorlayer from the side surface; monitoring a reflectance of each of theoxidizable regions of the at least two kinds of monitor-usesemiconductor layers or its variation and obtaining, from results ofmonitoring, an oxidization terminating time at which oxidization of theIII-V semiconductor layer in the laser portion should be terminated; andterminating oxidization of the III-V semiconductor layer at theoxidization terminating time to thus define an aperture that is anon-oxidized region of the III-V semiconductor layer. According toanother aspect of the present invention, a method of fabricating asurface emitting semiconductor laser includes the steps of: forming alaminate of semiconductor layers including a III-V semiconductor layercontaining Al on a substrate; forming, from the laminate, first, secondand third mesas of different sizes on the substrate so that sidesurfaces of III-V semiconductor layers included in the first throughthird mesas are exposed; oxidizing the III-V semiconductor layers of thefirst through third mesas from the side surfaces thereof; opticallymonitoring oxidized conditions on the III-V semiconductor layers of thesecond and third mesas and obtaining, from times when the III-Vsemiconductor layers of the second and third mesas are terminated aswell as the sizes thereof, an oxidization terminating time whenoxidization of the III-V semiconductor layer of the first mesa should beterminated;

-   -   and terminating oxidization of the III-V semiconductor layer of        the first mesa to thus form a current confinement layer        including an aperture that is a non-oxidized region of the III-V        semiconductor layer of the first mesa.

According to yet another aspect of the present invention, a method offabricating a surface emitting semiconductor laser includes the stepsof: forming, on a substrate, multiple monitor-use semiconductor layershaving stripes radiating from a center of the substrate, and a laserportion that includes semiconductor layers and is located on theperiphery of the multiple monitor-use semiconductor layers; monitoringoxidized conditions on the multiple monitor-use semiconductor layerswhen a selectively oxidized region is formed in the laser portion; andcontrolling oxidization of the selectively oxidized region on the basisof the oxidized conditions thus monitored.

According to a further aspect of the present invention, a method offabricating a surface emitting semiconductor laser includes the stepsof: forming, on a substrate, multiple monitor-use semiconductor layershaving stripes radiating towards ends of the substrate, and a laserportion that includes semiconductor layers and is located adjacent tothe multiple monitor-use semiconductor layers; monitoring oxidizedconditions on the multiple monitor-use semiconductor layers when aselectively oxidized region is formed in the laser portion; andcontrolling oxidization of the selectively oxidized region on the basisof the oxidized conditions thus monitored.

According to a still further aspect of the present invention, a surfaceemitting semiconductor laser includes: a substrate; and a laminate ofsemiconductor layers on the substrate, the semiconductor layersincluding a first reflection mirror of a first conduction type, anactive region on the first reflector mirror, a current confinementregion including an oxidized region, and a second reflection mirror of asecond conduction type, a mesa ranging at least from the secondreflection mirror to the current confinement layer, the mesa extendingat least from the second reflection mirror to the current confinementlayer, the oxidized region of the current confinement layer utilizing anoxidization terminating time obtained by detecting times when oxidizableregions included in at least two kinds of monitor-use semiconductorlayers having different shapes are totally oxidized.

According to another aspect of the present invention, an apparatus forfabricating a surface emitting semiconductor laser includes: aprojection part projecting light onto an area including at least twokinds of monitor-use mesas respectively including III-V semiconductorlayers that contain Al and have side surfaces exposed, when the sidesurfaces and a side surface of a III-V semiconductor layer of alaser-use mesa that contains Al and a side surface exposed are oxidized;a photoelectric conversion part converting reflected lights from themonitor-use mesas into electrical signals; an oxidization terminatingtime estimation part that detects times when the III-V semiconductorlayers of the two kinds of monitor-use mesas are totally oxidized on thebasis of the electrical signals, and estimates an oxidizationterminating time when oxidization of the III-V semiconductor layer ofthe laser-use mesa should be terminated; and an oxidization control partcausing oxidization of the III-V semiconductor layer of the laser-usemesa to be terminated at the oxidization terminating time to thus definea current confinement layer in the laser-use mesa.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1A is a cross-sectional view of a surface emitting semiconductordevice according to a first embodiment of the present invention, whereinthe cross-sectional view is taken along a line X-X shown in FIG. 1B;

FIG. 1B is a plan view of the surface emitting semiconductor laser shownin FIG. 1A;

FIGS. 2A, 2B and 2C are cross-sectional views of steps of a method offabricating the surface emitting semiconductor laser shown in FIGS. 1Aand 1B;

FIGS. 3A, 3B and 3C are cross-sectional views of steps of a method offabricating the surface emitting semiconductor laser shown in FIGS. 1Aand 1B; FIG. 4 illustrates oxidized conditions on an AlAs layer of alaser-use mesa and oxidized conditions of an AlAs layer of a monitor-usemesa;

FIG. 5 is a graph of a relation between an average reflectance involvedin a monitor-use stripe pattern and oxidization time;

FIG. 6 is a block diagram of an oxidization control apparatus accordingto an embodiment of the present invention;

FIG. 7 is a graph illustrating an average reflectance vs. oxidizationtime characteristic measured when two kinds of monitor-use mesas areused;

FIG. 8A is a graph showing accuracy of oxidization distance d3 obtainedwhen two kinds of monitor-use mesas according to an embodiment of thepresent invention are used;

FIG. 8B is a graph showing accuracy of oxidization distance obtainedwhen a single kind of monitor-use mesa is used;

FIGS. 9A and 9B are respectively plan views of variations of the twokinds of monitor-use mesas; FIGS. 10A and 10B are respectively planviews of radiating stripe patterns according to a second embodiment ofthe present invention;

FIGS. 11A and 11B are plan views of variations of the radiating stripepatterns shown in FIGS. 10A and 10B;

FIGS. 12A, 12B and 12C are plan views of other variations of theradiating stripe patterns shown in FIGS. 10A and 10B; FIG. 13 is a graphshowing a relation between the wavelength of light projected onto aconventional monitor-use sample and reflectance;

FIG. 14 is a graph showing an oxidization control method that is carriedout when the conventional monitor-use sample is used; and

FIG. 15 is a graph showing a relation between the reflectance andoxidization time obtained when the conventional monitor-use sample isused.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given of embodiments of the present inventionwith reference to the accompanying drawings.

FIG. 1A is a cross-sectional view of a surface emitting semiconductorlaser according to a first embodiment of the present invention, and FIG.1B is a plan view thereof. The present embodiment is a selectiveoxidization type surface emitting semiconductor laser 100 equipped witha laser portion 101 having a cylindrical mesa structure (post structureor pillar structure). A protection film that covers the laser portion(mesa) 101 and a bonding pad portion that extends from a metal contactlayer are omitted from illustration for the sake of simplicity.

The laser 100 has an n-type GaAs substrate 1, on which an n-type bufferlayer 2 is provided. An n-type lower DBR (Distributed Bragg Reflector) 3is provided on the buffer layer 2. On the lower DBR 3, laminated are anundoped lower spacer layer 4, an undoped quantum well active layer 5,and an undoped upper spacer layer 6 in this order. An active region 7 isformed so as to include the layers 4, 5 and 6. A p-type upper DBR 8 anda p-type contact layer 9 are laminated on the active region 7 in thisorder. The lowermost layer of the upper DBR 8 is a p-type AlAs layer 10,which serves as a current confinement layer equipped with a circularaperture 21 surrounded by an oxidized region.

A contact layer 9 is formed on the upper DBR 8. An interlayer insulationfilm 12 covers the bottom and side surfaces of the mesa as well as partof the top surface thereof. A metal layer 13 is formed on the interlayerinsulation film 12. The metal layer 13 makes an ohmic contact with thecontact layer 9, and serves as a p-side electrode for injecting currentto the laser portion. An n-type backside electrode 14 is provided on theback surface of the substrate 1. A circular aperture is formed in themetal layer 13 so as to be located in the center thereof. This apertureserves as an outgoing window 11 via which laser is emitted outwards. Thecenter of the outgoing window 11 is aligned with the aperture of thecurrent confinement layer 10, and substantially coincides with theoptical axis that extends from the substrate vertically and passesthrough the center of the laser portion 101. The lower DBR 3 is amultiple laminate of n-type Al_(0.9)Ga_(0.1)As layers andAl_(0.3)Ga_(0.7)As layers, each of which has a thickness λ/4n_(r) whereλ is the oscillation wavelength and n_(r) is the refractive index of themedium. The paired layers having different composition ratios arealternately laminated to a thickness of 40.5 periods.

The carrier concentration of the lower DBR 3 is 3×10¹⁸ cm⁻³ aftersilicon that is an n-type impurity is doped.

In the active region 7, the lower spacer layer 4 is an undopedAl_(0.6)Ga_(0.4)As layer. The quantum well active layer 5 includes anundoped Al_(0.1)Ga_(0.89)As quantum well layer and an undopedAl_(0.3)Ga_(0.7)As barrier layer. The upper spacer layer 6 is an undopedAl_(0.6)Ga_(0.4)As layer.

The upper DBR 8 is a multiple laminate of p-type Al_(0.9)Ga_(0.1)Aslayer and p-type Al_(0.3)Ga_(0.7)As layers, each having a thicknessλ/4n_(r) where λ is the oscillation wavelength and n_(r) is therefractive index of the medium. The paired layers having differentcomposition ratios are alternately laminated to a thickness of 30periods.

The carrier concentration of the upper DBR 8 is 3×10¹⁸ cm⁻³ after carbonthat is a p-type impurity is doped.

The p-type contact layer 9 is a GaAs layer and is 20 nm thick. Thecarrier concentration of the p-type contact layer 9 is 1×10²⁰ cm⁻³ aftercarbon that is a p-type impurity is doped. The metal layer 13, whichserves as the p-side electrode, is a laminate of Ti/Au.

Although not shown in FIGS. 1A and 1B, in order to reduce the seriesresistance of the laser portion, practically, an intermediate (graded)layer having an intermediate mixed crystal ratio of GaAs/AlAs betweenthe p-type Al_(0.9)Ga_(0.1)As layer and the p-type Al_(0.3)Ga_(0.7)Aslayer may be provided on the upper DBR 8 or below the lower DBR 3.

A description will now be given of a method of fabricating the surfaceemitting semiconductor laser shown in FIGS. 1A and 1B. In the presentembodiment, a monitor-use stripe pattern having semiconductor layers isformed in the center of the substrate 1, and semiconductor layers forlaser portions are arranged around its periphery. In the figuresdescribed blow, physical relations mentioned above are omitted, and aprocess for simultaneously forming semiconductor layers for laserportions and monitor-use semiconductor layers on the substrate 1 areschematically illustrated.

As shown in FIG. 2A, the buffer layer 2, the lower DBR 3, the activeregion 7, the upper DBR 8 and the contact layer 9 are laminated on thesubstrate 1 in this order. As shown in FIG. 2B, silicon oxide layers201, 201 a and 201 b are formed on the contact layer 9 by patterning.The silicon oxide layer 201 is a mask layer for forming the mesa of thelaser portion 101. The silicon oxide layers 201 a and 201 b are masklayers for forming mesas that are semiconductor layers for use inmonitoring. The mesa of the laser portion 101 is a cylindrical pillar,while the monitor-use semiconductor layers are like a slender stripe.The silicon oxide layer 201 is shaped into a circular pattern, and thesilicon oxide layers 201 a and 201 b are shaped into slender patterns.The cross section of FIG. 2B shows the short-side widths of the siliconoxide layers 201 a and 201 b, wherein the short-side width of thesilicon oxide layer 201 b is shorter than that of the silicon oxidewidth 201 a. The widths of the silicon oxide layers 201 a and 201 b areshorter than the diameter of the silicon oxide layer 201.

As shown in FIG. 2C, a laminate ranging from the contact layer 9, theupper DBR 8, the active region 8 and part of the lower DBR 3 isanisotropically etched by reactive ion etching (RIE) with a mixed gas ofboron trichloride and chlorine (BCl₃ and Cl₂). This anisotropic etchinguses the silicon oxide films 201, 201 a and 201 b as masks, and definesa laser-use mesa 210 and monitor-use mesas 210 a and 210 b. It is notnecessarily required to cause etching to advance up to the lower DBR 3but to expose at least the side surface of the AlAs layer 10. Forexample, etching of up to the active layer 5 of the active region 7 maybe accepted.

The mesa 210 of the laser portion 101 is like a cylindrical pillar, andthe monitor-use mesas 210 a and 210 b are like slender rectangularparallelepipeds. The long-side widths of the mesas 210 a and 201 b aremuch longer than the short-side widths thereof. The two mesas 210 a and210 b having the different widths are paired, and multiple pairs ofmesas are arranged at given pitches in the center of the substrate 1 soas to form straight stripes.

Next, oxidization of the AlAs layer (current confinement layer) 10 ofthe mesa 210 is started (FIG. 3A). This oxidization uses wet oxidizationin which a water vapor obtained by bubbling pure water heated to 95° C.is transported to a wet oxidization chamber with nitrogen being used ascarrier gas. The substrate is put in the oxidization chamber in advanceand the in-chamber temperature is set at approximately 340° C. The AlAslayers included in the mesas 210 and 210 a and 210 b are oxidized fromthe side surfaces thereof.

In the present embodiment, in order to control the oxidization reactionon the AlAs layer 10, namely, the size of the aperture 21 thereof,oxidized conditions of the AlAs layers 10 a and 10 b of the mesas 210 aand 210 b are monitored, and the terminating time of oxidization of theAlAs layer 10 is estimated on the basis of the oxidized conditions ofthe AlAs layers 10 a and 10 b. This control may be accomplished by anoxidization control apparatus 600 shown in FIG. 6. The apparatus 600includes a lamp 601, a spectroscope 602, a photodetector 603, and acontrol unit 604. The lamp 601 projects light in the wavelength range of400-1100 nm onto the area including the stripe pattern of the mesas 210a and 210 b on the substrate 1. The spectroscope 602 optically splitsreflected light from the stripe pattern into given wavelengths. Thephotodetector 603 is a light-receiving element such as a photodiode, andconverts split lights into electrical signals. The control unit 604monitors the signals from the photodetector 603, and controlsoxidization of the AlAs layer of the mesa 210.

When AlAs (or AlGaAs) is oxidized, AlO_(x) is formed in the oxidizedportion, which is an insulator, and has a reflectance different fromthat of AlAs. For instance, the average reflectance in the wavelengthrange of 800-1000 nm is 0.45 for AlAs and is 0.58 for AlO_(x). Thus, bymeasuring the average reflectance of the AlAs layer in progress ofoxidization, it is possible to track the degree of progress ofoxidization reaction on the AlAs layer.

FIG. 4 shows various oxidized conditions of the AlAs layer 10 of themesa 210 that forms the laser portion 101, and various oxidizedconditions of the AlAs layers 10 a and 10 b of the monitor-use mesas 210a and 210 b. FIG. 5 is a graph of an average reflectance vs. oxidizationtime characteristic of a stripe pattern for use in monitoring.

The planer pattern (oxidizable region) of the AlAs layer 10 of the mesa210 for the laser portion 101 has a circular shape, and has a diameter Dprior to oxidization.

The planer pattern (oxidizable region) of the AlAs layer 10 a of themonitor-use mesa 210 a has a width W2 on the short side and a height(length) H on the long side. Similarly, the planer pattern of the AlAslayer 10 b of the monitor-use mesa 210 b has a width W1 on the shortside and the same height (length) H on the long side as that of themonitor-use mesa 210 a. These patterns have a relation such thatW1<W2<D, W1<<H, and W2<<H.

In FIGS. 4 and 5, at the oxidization starting time T0, oxidization ofthe AlAs layers 10, 10 a and 10 b of the mesas 210, 210 a and 210 b hasnot yet been started, and the average reflectance from the stripepattern is at an initial value R0. The planer patterns of the AlAslayers 10, 10 a and 10 b are shown as blank areas in FIG. 4. Afteroxidization starts, oxidization of the AlAs layer 10 of the mesa 210evenly advances inwards from its side surface. In FIG. 4, oxidized areasare illustrated with hatching. Simultaneously, oxidization of the AlAslayers 10 a and 10 b of the mesas 210 a and 210 b evenly advancesinwards from the side surfaces thereof. Since the heights H of themonitor-use mesas 210 a and 210 b are much longer than the widths W1 andW2, it can be considered that oxidization of the AlAs layers 10 a and 10b advances from the opposing long sides H of the mesas 210 a and 210 b.The AlAs layers 10, 10 a and 10 b are oxidized at the same rate ofoxidization from time T0. When the oxidizable region of the AlAs layer10 b of the mesa 210 b is totally oxidized, in other words, when thewidth W2 of the mesa 210 b is totally oxidized, a unique waveform thatreflects the ratio of variation of the reflectance from the stripepattern for monitoring is observed. Oxidization of the mesa 210 b doesnot advance any more. This results in a turning point R1 at whichinclination of the average reflectance becomes small. The turning pointR1 takes place at time T1. At time T1, oxidization of the AlAs layer 10a of the mesa 210 a has advanced for a distance of W1/2 from theopposing sides, and a non-oxidized region (W2-W1) remains in the centerthereof. In FIG. 4, the region that has been oxidized is illustrated asa hatched area, and the non-oxidized region is illustrated as a blankarea.

Oxidization of the mesa 210 of the laser portion 101 has advanced for adistance of W1/2 from the sidewall thereof, and a non-oxidized region ofan aperture D1 (=D-W1) remains in the center thereof.

When the AlAs layer 10 a of the mesa 210 a is totally oxidized, aturning point R2 takes place at which the inclination of the averagereflectance from the stripe pattern of the monitor-use mesa 210 afurther changes. This is because oxidization of the mesa 210 a iscompleted and does not advance any more. The turning point R2 takesplace at oxidization time T2. At time T2, oxidization of the mesa 210 ofthe laser portion 101 has advanced for a distance of oxidizationdistance W2/2 from the sidewall thereof, and a non-oxidized region of anaperture D2 (=D-W2) remains at the center. According to an aspect of thepresent invention, oxidization terminating time T3 for forming designedoxidization distance d3 (namely, an aperture having diameter D3) in theAlAs layer 10 on the basis of the times T1 and T2 at which the turningpoints R1 and R2 of the average reflectance are obtained.

The widths W1 and W2 of the mesas 210 a and 210 b for use in monitoringare known, and the times T1 and T2 can be obtained from the turningpoints R1 and R2.

Thus, it can be seen that oxidization advances for a distanceΔd=(W2-W1)/2 during a time ΔT=T2-T1. It will be noted that “½” isprovided taking into consideration that oxidization simultaneouslyadvances from the both sides of each mesa. The rate of oxidization ofthe AlAs layer can be obtained from the time ΔT and oxidization distanceΔd. From the above relation, time T3 at which oxidization advances tothe designed oxidization distance d3 can be obtained as follows:T 3=T 2+ΔT/Δd(d 3−W 2/2).By terminating the oxidization process when time T3 elapses, it ispossible to form the aperture 21 having the designed oxidizationdistance d3 or dimensional accuracy D3 in the mesa 210 on the laserportion 101. The above-mentioned oxidization control process is carriedout by the control apparatus 604, and is preferably programmed in theform of software stored therein.

After the oxidization process, as shown in FIG. 3A, part of the AlAslayer 10 of the mesa 210 is selectively oxidized, and the aperture 21 isformed in the center of the current confinement layer 10.

Turning to FIG. 3B, the silicon oxide layers 201, 201 a and 201 b usedas the etching masks are removed, and the interlayer insulation film 12is formed so as to form the mesa 210. Of course, it is not necessary toapply subsequent processes to the monitor-use mesas 210 a and 201 b inthe same manner as the mesa 210.

Then, the aperture 12 a is formed in the interlayer insulation film 12on the top of the mesa 210, and the metal layer 13 is provided thereon.An aperture serving as the outgoing window 11 is formed in the center ofthe metal layer 13, and the n-side backside electrode 14 is formed onthe back surface of the substrate 1.

According to the present embodiment, two kinds of mesa-likesemiconductor layers having different widths (W1, W2) are provided foruse in monitoring oxidization, and attention is paid to unique waveformchanges of the reflectance when the mesas 210 a and 210 b have beentotally oxidized. The detection of the unit waveform change makes itpossible to accurately monitor and track the progress of oxidization onthe monitor-use mesas.

FIG. 7 shows measurement results obtained when the two kinds ofmonitor-use mesas are actually employed. The two kinds of mesas thatform stripe patterns having widths W1 of 9 μm and 18 μm, respectively,and are subject to an oxidization control in which the oxidizationtemperature is set at 370° C. and the oxidization distance d3 is setequal to 11 μm. The two different mesas form a pattern of a pair ofstripes. Multiple pairs of stripes are disposed in the center of thesubstrate 1. The aforementioned oxidization control apparatus 600initiates the oxidization process for the substrate 1, and commences toproject white light from the lamp 601 onto the area including the pairsof stripes arranged in the center of the substrate 1. Reflected lightfrom the substrate 1 is split into lights of the given wavelengths bythe spectroscope 602, which lights are then monitored.

In FIG. 7, the vertical axis denotes the reflectance, and the horizontalaxis denotes the oxidization time. The reflectance of the vertical axismay be replaced with the output of an equivalent electrical signalobtained by photoelectric conversion of a selected one of the splitlights. It can be seen from FIG. 7 that the first explicit change of thereflectance after initiation of oxidization takes place when themonitor-use mesa having the comparatively small width W1 (equal to 9 μm)has just been oxidized totally (time T1), and that the second explicitchange of the reflection occurs when the other monitor-use mesa havingthe comparatively large width W2 (equal to 18 μm) has just been oxidizedtotally (time T2).

It is possible to determine which one of the split lights or wavelengthsfrom the spectroscope 602 should be selected to monitor the reflectanceof its change by taking into account the patterns, sizes and laminatestructures of the monitor-use mesas or oxidization environment.Preferably, one of the split lights or wavelengths that exhibitsremarkable change in the reflectance should be selected.

FIG. 8A shows the accuracy of the oxidization distance d3 obtained whenthe two kinds of monitor-use mesas according to the present embodimentare used, and FIG. 8B shows the accuracy of the oxidization distanceobtained when only one monitor-use mesa is used. The horizontal axes ofFIGS. 8A and 8B denote the lot numbers of substrates subject to a batchprocess, and the vertical axes thereof denote normalized oxidizationdepths formed in the mesa 210. As is seen from FIG. 8A, the oxidizationdistance formed in the mesa 210 is regulated within a range of about 2%,and the standard deviation p is equal to 0.99%. In contrast, in theconventional oxidization control shown in FIG. 8B, the oxidizationdistance (which corresponds to the oxidization distance d3) is regulatedwithin a broader range of about 5%, and the standard deviation ρ isequal to 2.35%. It can be seen from the above that the use of the twokinds of monitor-use mesas for estimating the oxidization distance d3from the respective oxidization terminating times makes it possible tomore accurately control the oxidization distance d3 or the aperture D3.

In the present embodiment, the stripes of the two kinds of mesas orsemiconductor layers for monitoring are formed on the substrate (orwafer) separate from the laser portion 101. Instead of theabove-mentioned stripes, it is possible to employ stripes respectivelyformed by cylindrical mesas. For example, the mesas 210 a and 210 b foruse in monitoring may be structured so that the AlAs layers 10 a and 10b have an oval, ellipse or polygonal planer shape. Similarly, the mesaof the laser portion 01 is not limited to the cylindrical post, but maybe a square or rectangular post. The present invention does notnecessarily need the straight stripes, but any pattern formed by a pairof stripes of the mesas 210 a and 210 b may be used.

FIGS. 9A and 9B respectively show variations of the monitor-use mesas.The two kinds of mesas for use in monitoring do not essentially requirephysical separation.

For instance, as shown in FIG. 9A, a mesa 220 for monitoring has apattern having regions of widths W1 and W2. It is also possible tointerpose another mesa 221 having the same pattern as that of the mesa220 between integrally formed stripes of the mesa 220 so that the widthsW1 and W2 of the mesa 220 face the widths W2 and W1 of the mesa 221,respectively. As shown in FIG. 9B, mesas 231 and 232 for monitoring arealternately arranged. As described above, even when the mesas have thesame pattern, these mesas can be arranged so as to substantially havetwo kinds of mesas for use in monitoring. In short, it is enough to haveat least two kinds of oxidization regions having different widths, sizesor dimensions.

Second Embodiment

A description will now be given of a second embodiment of the presentinvention. In the foregoing, the monitor-use mesas are arranged so as toform a stripe pattern in the center of the substrate. In contrast,according to the second embodiment of the present invention, a stripepattern 701 that has stripes radiating from the center of the substrate1 is used, as shown in FIG. 10A. The radial stripe pattern 701 includesmultiple pairs of mesas, each pair having two kinds of mesas havingdifferent widths W1 and W2 as in the case of the first embodiment.Multiple mesas 702 that respectively form laser portions are arrangedaround the stripe pattern 701.

In the process following the formation of mesas, an etching mask ofresist in the photolithographic process is used to define the aperture12 a of the interlayer insulation film 12 and the outgoing window 11 ofthe metal layer 13. The process of forming the resist includes spincoating in which resist is dropped towards the substrate 1 that isrotating. In this spin coating, resist dropped on the substrate may movesmoothly along the stripe pattern 701 of the stripes radiating from thecenter of the substrate 1 and having symmetry with respect to thecenter. It is therefore possible to deposit the resist having an evenfilm thickness over the substrate 1. By enhancing the accuracy of theresist mask, it is possible to improve the accuracy of positioning theaperture 12 a and the outgoing window 11 and align them with the opticalaxis of the laser portion 101.

The present invention is not limited to the two kinds of mesas havingdifferent widths, but may use a monitor-use pattern of radial stripseach having the same width.

FIGS. 11A and 11B show modified examples of the monitor-use mesas. Aradial stripe pattern 801 shown in FIG. 11A is a modification of theradial stripe pattern 701 shown in FIG. 10A. The radial pattern 801shown in FIG. 1A includes isolated patterns 801 a, each of which isinterposed between two stripes radiating from the center. The patterndensity on the outer side of the radial stripe pattern can be increasedby the isolated patterns 801 a. An increased pattern density on theouter side of the radial stripe pattern reduces the bottom areas of themesas, and light reflected from the stripe pattern includes reducedreflected light from the bottom areas of the mesas, so that noisecontained in the signal can be reduced and error in the reflectancemeasurement can be reduced.

A semi-radial stripe pattern 802 shown in FIG. 11B may be used. Thesemi-radial stripe pattern 802 has divided groups, each of which has acenter angle of 45° and includes a pattern of straight stripes. Thepatterns of straight stripes of the divided groups are arranged so as toform an angle of 45°. For example, a pattern 802 a includes straightstripes extending at an angle of 45° with respect to an orientation flat803 of the wafer or substrate, and a pattern 802 b includes straightstripes extending in parallel with the orientation flat 803. The dividedgroups are arranged so that the straight stripes of the adjacent groupsform an angle of 45°. The semi-radial pattern 802 has reduced dispersionof the stripe width as compared to the radial pattern shown in FIG. 9A,and has an extremely increased pattern density as compared to the radialpattern shown in FIG. 8A.

FIGS. 12A through 12C respectively show variations of the radial stripepattern. A pattern shown in FIG. 12A includes stripe patterns 810, 811,812 and 813, which are arranged on the substrate 1 with rotationalsymmetry. Each of the stripe patterns 810-813 includes stripes 810 aradiating from the center of the substrate 1 and stripes 810 b extendingin the direction perpendicular to the orientation flat 803 of thesubstrate 1. A pattern shown in FIG. 12B includes four right-anglepatterns 820 with rotational symmetry. A pattern shown in FIG. 12C hasfour 45° inclined patterns 830 with rotational symmetry. Any of thepatterns shown in FIGS. 12A through 12C bring about the same effects asthe radial patterns. The straight stripes of the patterns 810 b, 820 and830 are not necessarily required to cross at the ends, but may be spacedapart from each other at a given distance. The monitor-use mesas thatform the patterns 810 b, 820 and 830 may be varied so as to have pairsof mesas arranged at given intervals, each of which pairs has two kindsof mesas having different widths. It is also possible to form a stripepattern with mesas each having the same width.

The present invention is not limited to the specifically describedembodiments, but other embodiments, variations and modifications may bemade without departing from the scope of the claimed invention.

Besides the cylindrical mesa is used for the laser portion 101 mentionedbefore, a mesa of a rectangular parallelepiped may be used. Themonitor-used mesas are not limited to the stripe type but may have anyshape.

In the foregoing, the monitor-use mesas are used for controllingoxidization of the current confinement layer 10. However, oxidizationcontrol is not limited to the use of the monitor-use mesas. Forinstance, light may be projected onto the mesa surface of the laserportion 101, and the oxidized condition on the current confinement layer(AlAs layer) may be monitored directly for oxidization control.

Besides the current confinement layer 10 of AlAs, a III-V semiconductorcontaining Al, such as AlGaAs, may be used. In the foregoing, the upperDBR 8 is of p-type and the lower DBR 3 is of n type. The aboveconduction types may be interchanged with each other. In a case wherethe outgoing light is extracted from the backside of the substrate 1,the upper DBR 8 is designed to have a larger number of layers than thelower DBR 3 and have a comparatively high reflectance.

The quantum well active layer is not limited to GaAs/AlGaAs-basedsemiconductors but may be made of, for example, GaAs/InGaAs-basedsemiconductors or GaAs/GaInNAs-based semiconductors. The emissionwavelength of light emitted from the quantum well layer is transparentto the GaAs substrate 1. This allows the outgoing light to be taken fromthe backside of the substrate 1 and brings about a process merit. In theforegoing, the contact layer 9 and the upper DBR 8 are handled asseparate layers in view of functions. However, the contact layer 9 ispart of the upper DBR 8.

In the foregoing, the n-side electrode 14 is formed on the backside ofthe substrate 1. Alternatively, the n-type electrode may be provided onthe semiconductor layer (for instance, the lower DBR 3) exposed in themesa bottom on the substrate.

Finally, the above description is summarized below.

According to an aspect of the present invention, the method offabricating a surface emitting semiconductor laser includes the stepsof: forming a laser portion of semiconductor layers and at least twokinds of monitor-use semiconductor layers on a substrate, the laserportion including a first reflection mirror layer of a first conductiontype, an active region, a III-V semiconductor layer containing Al and asecond reflection mirror of a second conduction type, the at least twokinds of monitor-use semiconductor layers having oxidizable regionshaving different shapes; etching the laser portion so as to form a mesaon the substrate in which a side surface of the III-V semiconductorlayer contained in the laser portion is exposed; starting oxidization ofthe III-V semiconductor layer from the side surface; monitoring areflectance of each of the oxidizable regions of the at least two kindsof monitor-use semiconductor layers or its variation and obtaining, fromresults of monitoring, an oxidization terminating time at whichoxidization of the III-V semiconductor layer in the laser portion shouldbe terminated; and terminating oxidization of the III-V semiconductorlayer at the oxidization terminating time to thus define an aperturethat is a non-oxidized region of the III-V semiconductor layer.

By using the two kinds of semiconductor layers having differentoxidizable regions, it becomes possible to accurately measure theturning points at which oxidization of the two kings of semiconductorlayers are respectively completed. It is further possible to obtaininformation about the oxidization times and oxidization distances of themonitor-use semiconductor layers. The information can be used toestimate the oxidization terminating time related to the aperture thatis to be formed in the III-V semiconductor layer of the laser portion.As a result, the aperture can be defined by highly reproducible processas designed.

Preferably, widths of the oxidizable regions of the at least two kindsof monitor-use semiconductor layers are smaller than a width of anoxidizable region of the III-V semiconductor layer of the laser portion,and the method further includes the steps of: obtaining a rate ofoxidization of the oxidizable regions of the monitor-use semiconductorlayers from variation in reflectance observed when the oxidizableregions of the monitor-use semiconductor layers are totally oxidized;and obtaining, from the rate of oxidization, a time when the aperturethat is the non-oxidized region of the laser portion should be formed.The reflectance of the monitor-use semiconductor layers or its variationappears explicitly. Therefore, the use of the reflectance or itsvariation contributes to detecting the oxidization rate more accuratelyand reducing error contained therein.

Preferably, the oxidizable regions of the at least two kinds ofmonitor-use semiconductor layers have widths W1 and W2 (W1<W2); thewidths W1 and W2 are smaller than a diameter D of the oxidizable regionof the III-V semiconductor layer of the laser portion; and the time whenthe aperture should be formed is obtained from the widths W1 and W2 andtimes T1 and T2 when the oxidizable regions of the at least two kinds ofmonitor-use semiconductor layers are totally oxidized. The relationbetween the oxidization time and rate can be obtained from themonitor-use semiconductor layers, so that the time necessary to definethe aperture of the non-oxidized region can be calculated byextrapolation.

Preferably, the at least two kinds of monitor-use semiconductor layershave mesas that are formed by etching at the same time as the mesa ofthe laser portion is formed; and the mesas of the at least two kinds ofmonitor-use semiconductor layers have a layer structure identical tothat of the mesa of the laser portion. Thus, the oxidization rates ofthe monitor-use semiconductor layers reflect the oxidization rate of thelaser portion. Thus, monitoring the status of advancing oxidization ofthe monitor-use semiconductor layers is equivalent to that of the laserportion. It is therefore possible to accurately control the oxidizationreaction on the III-V semiconductor layer containing Al.

Preferably, the at least two kinds of monitor-use semiconductor layersare arranged so as to form a strip pattern on the substrate. Thisarrangement averages dispersion of the shapes of the semiconductorlayers and that of the oxidization distances, and realizes more reliablemonitoring of oxidization condition.

For example, the mesa of the laser portion has a post shape, and theIII-V semiconductor layer containing Al is one of an AlAs layer and anAlGaAs layer.

According to another aspect of the present invention, the method offabricating a surface emitting semiconductor laser includes the stepsof: forming a laminate of semiconductor layers including a III-Vsemiconductor layer containing Al on a substrate; forming, from thelaminate, first, second and third mesas of different sizes on thesubstrate so that side surfaces of III-V semiconductor layers includedin the first through third mesas are exposed; oxidizing the III-Vsemiconductor layers of the first through third mesas from the sidesurfaces thereof; optically monitoring oxidized conditions on the III-Vsemiconductor layers of the second and third mesas and obtaining, fromtimes when the III-V semiconductor layers of the second and third mesasare terminated as well as the sizes thereof, an oxidization terminatingtime when oxidization of the III-V semiconductor layer of the first mesashould be terminated; and terminating oxidization of the III-Vsemiconductor layer of the first mesa to thus form a current confinementlayer including an aperture that is a non-oxidized region of the III-Vsemiconductor layer of the first mesa.

Preferably, the first mesa has a post shape, and the non-oxidized regionhas a diameter of approximately 3 μm. Since the oxidization terminatingtime can be estimated accurately, the non-oxidized region, namely, theaperture that is as small as approximately 3 μm can be realized. It istherefore possible to realize single-mode oscillation.

According to yet another aspect of the present invention, the method offabricating a surface emitting semiconductor laser includes the stepsof: forming, on a substrate, multiple monitor-use semiconductor layershaving stripes radiating from a center of the substrate, and a laserportion that includes semiconductor layers and is located on theperiphery of the multiple monitor-use semiconductor layers; monitoringoxidized conditions on the multiple monitor-use semiconductor layerswhen a selectively oxidized region is formed in the laser portion; andcontrolling oxidization of the selectively oxidized region on the basisof the oxidized conditions thus monitored. The monitor-use semiconductorlayers radiate from the center of the substrate. This facilitates tosmooth movement of resist that is spin-coated and achieves auniform-thickness resist film.

Preferably, the stripes radiating from the center are arranged withrotational symmetry about the center. The stripes radiating from thecenter may be divided into multiple groups at a given center angle, andeach of the multiple groups has straight stripes. The stripes includedin the adjacent groups among the multiple groups may form an angle ofapproximately 45°. For example, the stripes are equally divided intoeight multiple groups at an angle of 45°. The divided groups may includestraight stripes. The stripe pattern may be formed by a single kind ofmonitor-use semiconductor layers, or multiple pairs of stripe pattern,each being composed of two kinds of monitor-use semiconductor layers.

According to a further aspect of the present invention, the surfaceemitting semiconductor laser includes: a substrate; and a laminate ofsemiconductor layers on the substrate, the semiconductor layersincluding a first reflection mirror of a first conduction type, anactive region on the first reflector mirror, a current confinementregion including an oxidized region, and a second reflection mirror of asecond conduction type, a mesa ranging at least from the secondreflection mirror to the current confinement layer, the mesa extendingat least from the second reflection mirror to the current confinementlayer, the oxidized region of the current confinement layer utilizing anoxidization terminating time obtained by detecting times when oxidizableregions included in at least two kinds of monitor-use semiconductorlayers having different shapes are totally oxidized.

The above laser has the oxidized region of the current confinement layerwith high dimensional accuracy. It is therefore possible to efficientlyconfine current and light and to expect stable performance as designed.

According to a still further aspect of the present invention, theapparatus for fabricating a surface emitting semiconductor laserincludes: a projection part projecting light onto an area including atleast two kinds of monitor-use mesas respectively including III-Vsemiconductor layers that contain Al and have side surfaces exposed,when the side surfaces and a side surface of a III-V semiconductor layerof a laser-use mesa that contains Al and a side surface exposed areoxidized; a photoelectric conversion part converting reflected lightsfrom the monitor-use mesas into electrical signals; an oxidizationterminating time estimation part that detects times when the III-Vsemiconductor layers of the two kinds of monitor-use mesas are totallyoxidized on the basis of the electrical signals, and estimates anoxidization terminating time when oxidization of the III-V semiconductorlayer of the laser-use mesa should be terminated; and an oxidizationcontrol part causing oxidization of the III-V semiconductor layer of thelaser-use mesa to be terminated at the oxidization terminating time tothus define a current confinement layer in the laser-use mesa.

It is therefore possible to control the oxidized region of the III-Vsemiconductor layer containing Al and realize highly reproducibleoxidized region with high dimensional accuracy. This achieves improvedproduction yield and cost reduction.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A surface emitting semiconductor laser comprising: a substrate; and alaminate of semiconductor layers on the substrate, the semiconductorlayers including a first reflection mirror of a first conduction type,an active region on the first reflector mirror, a current confinementregion including an oxidized region, and a second reflection mirror of asecond conduction type, a mesa ranging at least from the secondreflection mirror to the current confinement layer, the mesa extendingat least from the second reflection mirror to the current confinementlayer, the oxidized region of the current confinement layer utilizing anoxidization terminating time obtained by detecting times when oxidizableregions included in at least two kinds of monitor-use semiconductorlayers having different shapes are totally oxidized.
 2. The surfaceemitting semiconductor laser as claimed in claim 1, wherein the currentconfinement layer is an AlAs layer.
 3. The surface emittingsemiconductor laser as claimed in claim 1, wherein the currentconfinement layer is an AlGaAs layer.